1. Field
The disclosed subject matter relates to an optical droplet sensor and method capable of optically detecting droplets attached to a surface of a light transmissive structure, for example, a windshield plate.
2. Description of the Related Art
An optical droplet sensor is conventionally installed on an inner surface of a windshield of a vehicle and detects droplets attached on an outer glass surface thereof for automatically operating wipers to ensure a fine view while driving in rain. An example of such an optical droplet sensor is described as follows.
This conventional sensor includes a beam transmitter 50 and a beam receiver 51 mounted on a printed circuit board 52 as shown in FIG. 6 such that respective optical axes in exit and entry directions cross each other. Two plane-convex lenses 53a, 53b are integrated through a coupling section 54 to form a prism lens 55, which is arranged such that optical axes of the plane-convex lenses 53a, 53b match the optical axes of the beam transmitter 50 and the beam receiver 51, respectively.
When the optical droplet sensor thus configured is installed on an inner surface 57 of a windshield glass 56, light emitted from the light emitter 50 is transformed into parallel light through the plane-convex lens 53a and introduced into the prism lens 55. The parallel light introduced into the prism lens 55 repeats reflections (total reflections) between an outer surface 58 of the windshield glass 56 and an outer circumferential surface 59 of the coupling section 54 and reaches the plane-convex lens 53b. The parallel light received at the plane-convex lens 53b is collected through the plane-convex lens 53b, then exits outward from the prism lens 55 and enters the beam receiver 51.
In general, the density of raindrops is lower at the beginning of rainfall. Even in such case, reliable detection of rainfall benefits from a wide raindrop detection region and an improvement in accuracy of raindrop detection. For that purpose, in the optical droplet sensor configured as described above, a device is applied to widen the raindrop detection region and to increase the number of reflections (total reflections) between the outer surface of the windshield glass and the outer circumferential surface of the coupling section in the prism lens. The device is directed to arrange both the beam transmitter and the beam receiver outside the prism lens. In this case, however, the shape/size of the raindrop sensor in the direction of the length disadvantageously becomes larger. In addition, the area of the raindrop detection region may still be insufficient, and thus it is difficult to perform high-accuracy droplet detection.
To address the above and other issues, another conventional optical droplet sensor can be used, which is configured as shown in FIG. 7. This sensor includes a beam transmitter 60 and a beam receiver 61 arranged in parallel and a prism lens 62 formed to be almost U-shaped in section. The sensor has opposite inner faces on arc-curved convex lenses 63a, 63b, which are formed by rotating arc shapes in section about the beam transmitter 60 and the beam receiver 61, respectively. The arc-curved convex lenses 63a, 63b have outer circumferential faces 64a, 64b, which are shaped to be parabolic.
When the optical droplet sensor thus configured is installed on an inner surface 66 of a windshield glass 65, light emitted from the light emitter 60 is transformed into parallel light through the arc-curved convex lens 63a and introduced into the prism lens 62. The parallel light introduced into the prism lens 62 travels toward the outer circumferential face 64a of the arc-curved convex lens 63a on the side close to the light emitter 60. It is then reflected (totally reflected) from the outer circumferential face 64a and travels toward an outer surface 67 of the windshield glass 65. The parallel light traveling toward the outer surface 67 of the windshield glass 65 is reflected (totally reflected) from the outer surface 67 of the windshield glass 65 and travels toward the outer circumferential face 64b of the arc-curved convex lens 63b on the side close to the light receiver 61. It is then reflected (totally reflected) from the outer circumferential face 64b and travels toward the arc-curved convex lens 63b on the side close to the light receiver 61. The parallel light received at the arc-curved convex lens 63b on the side close to the light receiver 61 is collected through the arc-curved convex lens 63b on the side close to the light receiver 61. It is then released outward from the prism lens 62 and enters the beam receiver 61 (for example, please see U.S. Pat. No. 6,627,910, which is hereby incorporated in its entirety by reference).
The optical droplet sensor thus configured has a narrowed interval between the beam transmitter and the beam receiver arranged in parallel to reduce the shape/size in the direction of the length. It has a function of receiving the light emitted from the beam transmitter and introduced into the prism lens and directing the light to the outer surface of the windshield glass. It also has a function of receiving the light reflected from the outer surface of the windshield glass and directing the light to the arc-curved convex lens on the side close to the light receiver. These functions are achieved by forming the respective outer circumferential faces of the arc-curved convex lenses as reflecting surfaces (total reflecting surfaces).
To make the outer circumferential face of the arc-curved convex lens function as the reflecting surface (total reflecting surface), the light emitted from the beam transmitter and introduced into the prism lens, and the normal to the outer circumferential face of the arc-curved convex lens are required to have an angle of intersection (an angle of incidence of light to the outer circumferential face of the arc-curved convex lens) larger than a critical angle. At the same time, the light reflected from the outer surface of the windshield glass, and the normal to the outer circumferential face of the arc-curved convex lens are required to have an angle of intersection (an angle of incidence of light to the outer circumferential face of the arc-curved convex lens) larger than a critical angle. Therefore, it is required to set a longer distance between each of the beam transmitter and the beam receiver and the outer circumferential face of the arc-curved convex lens in the direction almost normal to the windshield glass. As a result, the shape/size in the direction of the height of the raindrop sensor becomes larger.